Process and Apparatus for Recovering Energy from Wastewater

ABSTRACT

The invention provides a process and apparatus for the recovery of heat energy from wastewater. Wastewater, for example grey water from a domestic residence, is introduced to a detention chamber, which provides effective decoupling between the introduction of new wastewater and the demand for heat energy from its ultimate application.  A heat exchange surface, in contact with the wastewater on one side and a working fluid on the other, extracts heat from the detention chamber through thermal conduction and the working fluid is transferred, via a heat pump, to a second heat exchange surface. The second heat exchange surface, in contact with the working fluid on one side and heat energy storage media on the other, transfers heat energy to the storage media through conduction. Heat energy can then be extracted from the storage media for applications including heating of potable water, or provision of building heating.

FIELD OF INVENTION

The present invention relates to a process and apparatus for the removalof residual heat from wastewater for reuse. The preferred embodimentdescribed as an example herein is particularly suited to the removal ofheat from grey water in domestic residences, for later reuse insupplying energy for water heating in the home.

BACKGROUND

Globally, two key factors reducing the environmental sustainability ofhousing are water consumption and energy use. Consumption of theseresources is increasing due to increasing population. Pollution isreducing the availability of fresh water, while efforts to reducepollution increase energy production costs. The problem is compounded bylocalized population growth in greenfield developments in sunbelt areas(where water resources and infrastructure are scarce) and by the effectsof a changing climate. With the energy embedded in potable water,through purification and distribution, accounting for a significantproportion of global energy generation—the ‘water-energynexus’—opportunity exists to target both problems in an integrated way.

Wastewater discharged from domestic premises is a notable loss of energyfrom the typical home. Reduced energy consumption through the recoveryor elimination of waste heat has seen increasing adoption, through heatrecovery ventilation (HRV) systems, low-e glass and improved insulation;yet wastewater heat has received little attention. It is a challenge fortwo reasons: the contaminant load will quickly block efficient, singlepass heat exchangers, and the irregular pattern of waste productionlimits the amount of heat that can be easily recovered. Incumbentsolutions simplify the process by targeting relatively clean showerwater (where warm waste and a requirement for delivery of hot watercoincide), but this does not tap the full potential for energy savings.

In situations where wastewater treatment is also desirable , the removalof heat from the wastewater stream presents additional challenges forthe treatment methodology. Biological systems, for example, rely on theaction of microorganisms to digest contaminants, but these processes areslowed as the temperature is reduced. Similarly, low temperatures reducethe efficiencies of many filtration and chemicals systems. Someflocculation processes, for example, fail completely outside an optimaltemperature range.

Newly developed wastewater treatment processes (such as PCT2010902814)eliminate this temperature dependence, enabling the integrated approachdescribed by this invention.

Existing methods of high efficiency water heating technologies includesolar collectors and air-sourced heat pump systems, but thesetechnologies suffer major flaws that have prevented their widespreadadoption. Solar applications operate only during daylight hours and aredependent on weather (producing reduced output during cloudy weather).While evacuated tube solar collectors are largely independent of outsidetemperature, they are vulnerable to hail. Alternatives can be subject tobursting during freezing temperatures. Air-sourced heat pumps present analternative, but their efficiency is highly dependent on outside airtemperature, making them unsuitable in cold climates. Passive systemsare also available and, while cost effective, they do not preformreliably, place waste and potable streams in close proximity, and do nottolerate highly contaminated waste streams.

There is, therefore, a need in many circumstances for a more reliable,robust form of heat capture device that can supply the domestic hotwater needs in an efficient way. It must also be compatible withwastewater treatment technology to ensure that direct energy consumptionfor water heating, and the flow-on effects of supplying water, can beaddressed.

SUMMARY OF THE INVENTION

In one respect, the current invention resides in a process for theremoval of heat energy from wastewater that comprises:

-   -   a) Collection of wastewater in a reservoir;    -   b) Transfer of heat energy from the collected wastewater into a        working fluid;    -   c) Transport of the working fluid;    -   d) Transfer of the heat energy from the working fluid to a heat        energy reservoir; and    -   e) Transfer of the heat energy, on an as-needed basis, from the        heat energy reservoir to the incoming potable water supply.

According to the embodiment described, the preferred method of heattransfer is using a heat pump, where the working fluid is a refrigerantgas. Many commercially available refrigerant gases are suitable,including R-134a, R-417a, R-744, R-600a, R-410a; with R-417a being themost preferred according to this invention.

Preferably, the process further comprises:

-   -   a) Passing the influent wastewater over a filter to remove        particles larger than approximately 200 μm;    -   b) Delivery of the wastewater, with heat energy removed, to a        wastewater treatment system for water recovery.

In another respect, the invention resides in an apparatus for theremoval of heat energy from wastewater. The apparatus comprises:

-   -   a) A device allowing seamless interconnection between the        wastewater supply and the process described herein;    -   b) A heat exchange surface in contact with the wastewater;    -   c) A heat storage reservoir containing heat storage media;    -   d) Heat exchange surfaces in contact with the heat storage        media;    -   e) Plumbing to transfer working fluid between the heat exchange        surfaces;

In the preferred embodiment, the apparatus further comprises:

-   -   a) A compressor to move the working fluid between the heat        exchange surfaces;    -   b) A thermostatic expansion valve, or a capillary tube, to        promote a phase change in the working fluid;    -   c) Temperature measuring devices;    -   d) A thermostatic control system;    -   e) A supplemental, backup heating device;    -   f) Phase change media in the heat storage reservoir; and    -   g) Plumbing connecting the potable water supply to the heat        storage device;

Additionally, the interconnection device may further comprise one ormore of the following:

-   -   a) A detention chamber to buffer and smooth peak loads on the        system;    -   b) A wastewater backflow prevention device;    -   c) A pump for the transfer of cooled wastewater to a treatment        system;    -   d) A level sensor to control the pump;    -   e) An integrated filter capable of removing particles larger        than approximately 200 μm;    -   f) A sealed lid to prevent the ingress of water;    -   g) A vent to encourage reliable flow of water;

The process according to this invention is ideally suited to thetreatment of domestic grey water. ‘Grey water’ is wastewater producedfrom fixtures including showers, hand sinks and laundry facilities.These fixtures are not designed for the collection of human excrement ordischarges, and faeces or urine does not grossly contaminate theresulting wastewater.

Grey water fixtures generally include major sources of hot waterconsumption, and have warmer resulting waste streams. It is importantthat these are captured to ensure the invention described hereinoperates at the highest efficiency.

DESCRIPTION OF DRAWINGS

The following drawings describe, in a non-limiting way, the inventionwith respect to a preferred embodiment:

FIG. 1. Describes the process of the invention and how it may beinstalled to capture heat energy from domestic grey water.

FIG. 2. Presents a detailed cross-section of an appropriateinterconnection device.

DETAILED DESCRIPTION OF THE INVENTION

The invention resides in a process for the recovery of heat energy fromwaste water. The invention is suitable to application to many types ofwastewater; preferably this water is generated in domestic residences.The invention can also be applied to other waste streams where warmwater is generated, and a need to heat incoming water coincide. Waterprocessed according to this invention must be above 0° Celsius, andpreferably wastewater should be generated between 15 and 65 degrees.

According to FIG. 1, a preferred embodiment is depicted in which theinvention is coupled to the grey water sources (100) in a domesticresidence (101). Wastewater is directed, via separated plumbing (102)designed to maintain separation between grey water and more heavilycontaminated streams, to an interconnection device (103). Theinterconnection device has a number of functions, including acting as abuffer to decouple the generation of wastewater from the treatment, heatcapture and reuse processes. Each of these processes operates accordingto a differing (regular or irregular) schedule, and the decouplingeffect allows them to be interconnected. Inside the interconnectiondevice, a heat exchange surface (104) is placed in contact with thecollected wastewater. The device additionally comprises connection tothe conventional sewer system (105) or an alternative method ofwastewater disposal such as a leach field, septic system, holding pondor aerated wastewater treatment system. A separate connection to asuitable grey water treatment system (106) is also included. Thisconnection might alternatively be used to supply grey water forapplications other than treatment and reuse.

The heat exchange surfaces act as a barrier between the grey water,contained in the interconnection device, and a refrigerant working fluidcontained by a network of refrigerant plumbing arranged in a closed loop(107). The plumbing can be made from a great number of materials, withcopper being the most preferred. The loop operates in such a way thatrefrigerant is expanded from a liquid phase, by means of an expansionvalve (108) or a capillary tube. The cold gas is passed through the heatexchange surface where its temperature is raised by energy transfer fromthe collected wastewater. This warm gas can then be compressed using thecompressor (108), where it is passed to another heat exchange surface(109). This heat exchange surface is in contact with heat storage media(110) in a heat storage reservoir (111). The hot refrigerant is able totransfer energy, via the heat exchange surface, into the media. Theremaining plumbing (112) then returns the refrigerant fluid to theexpansion valve and the cycle is repeated.

If a second heat exchange surface (113) is placed in contact with theheat storage media, energy can be transferred to cold incoming potablewater (114). This system operates most efficiently when there is a highdifference in temperature between the storage media and the incomingwater supply. For this reason, the heat storage reservoir will beinsulated and will preferably contain some form of supplementary heatsource (115). This heat source may be of any available type, including:electric element, gas fire, solar, geothermal, wood fire, among othersas appropriate. If the storage media is water, it is desirable that thetemperature be maintained above 65° Celsius in order to prevent thegrowth of thermophillic bacteria. The water must also be prevented fromboiling, by remaining below 100° Celsius, to prevent undue pressure onthe structure of the reservoir. A reservoir containing water wouldrequire a pressure relief valve. Alternatively, the heat storagereservoir may contain a phase change material, such as paraffin wax,capable of storing larger amounts of heat energy for a given volume.

In some circumstances, it is desirable that the heat energy storagemedia (110) is itself the potable water supply. In this instance, theheat exchange surface (113) is replaced by an open pipe allowing directflow of potable water into, and out of, the reservoir. For applicationsof this type, a tempering valve (116) is placed between the heat energyreservoir and any residential fixtures. This ensures water supplied tothe home is maintained at a temperature suitable for application tohuman skin.

The invention further resides in an apparatus for the recovery of energyfrom wastewater. The apparatus facilitates the extraction of heat energyfrom water streams by providing an interconnection between heat transferequipment and the waste that would be sent directly to sewer in ordinarycircumstances. The apparatus also provides a connection betweenincoming, cold, potable water and heat stored in an energy storagereservoir, along with a mechanism to transport heat energy between theinterconnection device and the reservoir. The heat transfer mechanismmight comprise a direct heat transfer, dilution of one stream withanother, conversion to mechanical or chemical energy, or conversion toelectrical energy. In the preferred embodiment, energy is transported bymeans of a refrigeration cycle, or heat pump.

According to FIG. 2, the interconnection device comprises a waste inlet(200), where wastewater, preferably including hot domestic grey water,enters the device. The interconnection device may be made of a widerange of suitable waterproof materials including plastic, metal,composites, or natural substances. Preferably, the material should besmooth, to reduce the adherence of wastewater contaminants and shouldhave low thermal conductivity, to reduce the loss of heat energy to thesurrounding environment. In many applications, the plumbing connectionwill be below the surface of the ground, so the design of the devicemust be such that it can withstand ground pressures. It must also havefeatures that prevent it being lifted from the soil due to upliftpressures, frost, or a water table that rises periodically. Thepreferred embodiment described here would be constructed from aninsulating plastic material (201) of sufficient thickness to attenuateheat losses to the surrounding soil.

Water entering via the inlet is then optionally passed across a filter(202) where particles larger than a prescribed size are separated fromthe main flow. The filter may be of many configurations, and may bewashable, disposable or self-cleaning. Preferably, a self cleaningfilter incorporating wedge wires designed to separate particles largerthan approximately 200 μm is used. Collected particles (203) are thendirected, using a small amount of incoming wastewater, toward the outlet(204).

Water, with larger particles removed, is then directed into a heatextraction chamber (205), where it is placed in contact with a heatexchange surface (206). The heat exchange surface is also in contactwith a refrigerant working fluid in the preferred embodiment. Theworking fluid is connected to the rest of the process by means ofplumbing with an inlet (207) and outlet (208). For best efficiency, itis preferable that the inlet is nearer the top of the chamber, and thehottest water, and the outlet nearest the bottom of the chamber.

Alternatively, the heat exchange surface could be directly in contactwith the potable supply, or could comprise an alternative method ofenergy transfer, like a Peltier device. The heat transfer chamber isdesigned such that newly incoming water contacts the heat exchangesurface before being directed into the main body of the tank. Thisensures that the temperature of water in contact with the heat exchangesurface is at the highest possible temperature, which promotes efficienttransfer into the working fluid, and thus efficient operation of theapparatus.

Domestic wastewater is generated in a pulsatile fashion, for example theemptying of a bath. For this reason, the heat extraction chamber issized such that it can contain pulses of wastewater for a sufficientperiod to extract the maximum amount of heat. Preferably, in a typicalhome, the size of the chamber is between 50 and 500 litres, with between100 and 150 litres being most preferred. Other applications will requirethe chamber to be sized differently as appropriate.

Temperature in the heat extraction chamber is maintained above acritical point through the use of a thermostatic control system (209)and a temperature-measuring device (210). In the case of water, thetemperature must be maintained above the freezing point, 0° Celsius, toensure that ice does not form. Ice reduces the efficiency of heattransfer by insulating the heat exchange surface from any incoming,warmer water. It can also block flows and disrupt the intended operationof the interconnection device. The low temperature threshold for thethermostatic control system is set between 0° and 4° Celsius, withbetween 2° and °4 degrees being the optimum range according to thisinvention. At temperatures below this threshold, the heat energyrecovery process is stopped until further wastewater is gathered and thetemperature returns above the threshold. To reduce starting and stoppingfrequency, it is desirable that the thermostatic control system includesa lag, for example: turning off the heat recovery equipment when thetemperature falls to 2° Celsius, and activating it again only when thetemperature rises to 4° C.

Water in the chamber can be directed through the use of baffles (211)and weirs (212) to ensure that the hottest water remains in contact withthe heat exchange surface for the maximum time possible. Optionally, itcan be directed by mechanical means, such as a recirculation pump ormechanical mixer arranged in such a way as to maintain turbulent flowand circulation within the chamber.

After the heat has been recovered, water is directed to a second storagechamber by passing over a final weir (212). This chamber (213) isdesigned to uncouple the pulsatile output of water generated by theresidence, from the processing requirements of a downstream wastewatertreatment system. In may cases, these systems operate in a continuousfashion, or have a defined batch size that would be otherwiseincompatible with the output from a typical home. The size of thechamber will depend, in particular, on the specific treatment processbeing used, but will typically be between 50 litres and 2000 litres.Most preferably, the detention chamber will have a capacity between 100litres and 300 litres. Water can be directed from the detention chamberto a suitable wastewater treatment system by means of a pump (214),optionally controlled by a level sensor (215), and interconnectingplumbing (216).

In some cases, water may not be transferred from the detention chamberat the same rate it is being generated by the residence. In these cases,the detention chamber may become full and water will exit the chambervia an overflow port (217), traveling directly to the conventional sewersystem, or other conventional method of wastewater disposal, via aplumbing connection (218). In these circumstances, the outgoingwastewater will carry away any remaining particulate matter that hasbeen collected by the filter.

Finally, the interconnection device is designed to prevent any dischargeof malodorous gases to the local environment. A lid (219) with a tightseal (220) also prevents the ingress of ground water, or rainwater. Someventilation is required to ensure the reliable flow of liquid throughthe device, so a connection to a standard plumbing vent is required. Thedevice may also include a backflow prevention system (221), designed toprevent waste from the conventional sewer system from entering thesystem by the reverse path and contaminating the grey water with heavilysoiled streams.

1. A process for recovering heat energy from wastewater comprising a. Collection of wastewater in a reservoir; b. Transfer of heat energy from the collected wastewater into a working fluid; c. Transport of the working fluid; d. Transfer of the heat energy from the working fluid to a heat energy reservoir; and e. Transfer of the heat energy, on an as needed basis, from the heat energy reservoir to an incoming water supply.
 2. The process of claim 1, wherein the process further comprises one or more of the following steps: a. Passing influent water over a filter to remove particles larger than approximately 200 μm; b. Delivery of the wastewater, with heat energy removed, to a wastewater treatment system for water recovery.
 3. The process of claim 1, wherein the incoming water supply is potable water suitable for human contact through washing or bathing.
 4. The process of claim 1, wherein the incoming water supply is potable water suitable for human consumption through drinking or food preparation.
 5. The process of claim 1, wherein the incoming water supply is for use in space heating.
 6. The process of claim 1, wherein transport of the working fluid is carried out by means of a heat pump.
 7. The process of claim 2, wherein the filter is periodically backwashed with purified wastewater.
 8. The process of claim 1, wherein the working fluid is a refrigerant gas.
 9. The process of claim 2 wherein the wastewater treatment system uses mechanical separation of impurities.
 10. The process of claim 2, wherein the wastewater is grey water.
 11. An apparatus for the removal of heat energy from wastewater comprising the following: a. A device allowing seamless interconnection between the wastewater supply and the process described herein; b. A heat exchange surface in contact with the wastewater; c. A heat storage reservoir containing heat storage media; d. Heat exchange surfaces in contact with the heat storage media; and e. Plumbing to transfer working fluid between the heat exchange surfaces.
 12. The interconnection device of claim 11, wherein the interconnection device includes a detention chamber.
 13. The interconnection device of claim 11, wherein the interconnection device includes a wastewater backflow prevention device.
 14. The interconnection device of claim 11, wherein the interconnection device includes a level sensor to control a pump.
 15. The interconnection device of claim 11, wherein the interconnection device includes a pump for the transfer of cooled wastewater to a treatment system.
 16. The interconnection device of claim 11, wherein the interconnection device includes an integrated filter.
 17. The interconnection device of claim 11, wherein the interconnection device includes a sealed lid to prevent the ingress of water.
 18. The interconnection device of claim 11, wherein the interconnection device includes a vent to encourage reliable flow of wastewater.
 19. The interconnection device of claim 16, wherein the filter is capable of removing particles larger than approximately 200 μm.
 20. The apparatus of claim 11, further comprising one or more of the following: a. A compressor to move the working fluid between the heat exchange surfaces; b. A device promoting phase change in the working fluid; c. Temperature measuring devices; d. A thermostatic control system; e. A supplemental, backup heating device; f. Phase change media in the heat storage reservoir; and g. Plumbing connecting the potable water supply to the heat storage device;
 21. The apparatus of claim 20, wherein the device promoting a phase change in the working fluid is a thermostatic expansion valve.
 22. The apparatus of claim 20, wherein the device promoting a phase change in the working fluid is a capillary tube.
 23. The apparatus of claim 20, wherein the incoming water supply is potable water suitable for human contact through washing or bathing.
 24. The apparatus of claim 20, wherein the incoming water supply is potable water suitable for human consumption through drinking or food preparation.
 25. The apparatus of claim 20, wherein the incoming water supply is for use in space heating. 